#65 X-Bogosity header filtered from attachment breaks signature

My bogofilter version 0.13.7.2 seems to filter any
existing X-Bogosity header not only from the header of
the email but also from the body, especially from the
header of an included message/rfc-822 attachment, i.e.
a forwarded mail. This causes PGP/MIME signatures
(created with Enigmail in my case) to break.

I believe that bogofilter schould leave the body of an
email as it is and only change the header. If you
disagree, this schould at least become an option.

I had a closer look at the code. There seem to be two places
where X-Bogosity headers are filtered. One is in
write_message, in main.c or passthrough.c, and this one
takes care of only processing the header.

The other is the yy_get_new_line in lexer.c, and this is the
one where things go wrong. We could just remove those 7
lines, which would also result in this header field being
fed into the lexer, which IMHO is not a bad thing.

If you really want to prevent this, you'd have to add
textblock_add calls to the skip_spam_header function, but I
see absolutely no reason why you should.

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